The World Health Organization said it may issue a much more stringent travel advisory on Asian cities with deadly pneumonia outbreaks. A tougher warning could seriously hurt the Asian economy.

World Health Organization officials say international travel does not need to be disrupted to contain the spread of Severe Acute Respiratory Syndrome, known as SARS.

But if the disease, which can cause a severe form of pneumonia, continues to spread, the U.N. agency will toughen its warnings. The agency cannot almost all of them in Asia, and more than 15 have died. The worst hit cities are Hong Kong, Singapore and Hanoi.

WHO already has warned that those who have traveled to areas hit by SARS should seek immediate help if they develop high fevers, severe flu-like symptoms and signs of pneumonia. The agency recommends that airlines watch for passengers with such symptoms.

"We will review the data. If there is any evidence to modify it, we will modify the advisory," said Dick Thompson, a WHO spokesman.

Singapore this week has quarantined at least 740 people who may have been exposed to SARS. The country has reported 65 SARS cases.

The disease may first have appeared a few months ago in southern China, where at least 300 people developed symptoms similar to those seen in SARS. WHO officials are in China studying the outbreak.

The WHO said Hong Kong is its biggest concern because of the daily rise in the number of cases. Almost 290 people have come down with SARS in Hong Kong. At least 10 have died.

The exact cause of the disease is not known. Researchers in Hong Kong, Europe, Australia and the United States have spotted several possible causes.

Until recently, officials said most SARS patients had a direct link to a handful of original patients in Hong Kong.

Liu Shao-Haei, an official with Hong Kong's Department of Health, said the government now is concerned SARS has spread to the wider community. "I think we have had cases in the community. It is alarming, very alarming," Mr. Liu said.

Already, concerns about SARS have cut sharply into Asia's tourism industry, adding to the damage done by fears linked to the war in Iraq. Hotels and airlines around the region are reporting a drop in bookings. In Hong Kong, some international rugby teams have withdrawn from the Rugby Sevens tournament, which begins Friday. The event normally draws several-thousand tourists.

A more stringent travel warning could be devastating to many Asian economies, particularly Singapore and Hong Kong, which are regional business and travel hubs. Many of the world's largest companies have their regional headquarters in Singapore and Hong Kong, and would find it difficult to do business if travel were restricted.